Gemara

(Rav Nachman): If two documents (of sale or gift for the same property to Ploni) have different dates, the latter is Mevatel (invalidates) the former.

Clearly, if the first document was a sale and the second is a gift, the second is not Mevatel the first. It was written to protect Ploni from the law of Bar Metzra. (When land is sold, the neighbors have first rights to buy it. This does not apply to gifts.)

All the more so, if the first was a gift, and the second was a sale, it does not Mevatel the first. The second was written for Acharayos (to guarantee Ploni compensation if the property will be taken by creditors).

Rishonim

Rif: If the first document was a sale and the second is a gift, the second is to evade the law of Bar Metzra. If the first was a gift and the second was a sale, the second is for compensation.

Rambam (Hilchos Zechiyah 5:8): If Ploni has two documents for one field with different dates, if the first was a gift and the second was a sale, the latter does not Mevatel the former. We can say that it was written to give Acharayos, even if it does not mention Acharayos, because Acharayos Ta'os Sofer (we assume that there is Acharayos, just the scribe omitted it). If the first document was a sale and the second is a gift, Ploni acquired from the date of the first. The second was written to evade the law of Bar Metzra.

Rosh (4:5, according to Hagahos ha'Bach): When the first was a sale and the second is a gift, usually we can say that it was to evade the law of Bar Metzra. The buyer shows only the latter. If he shows both, it is clear that the latter is merely to give better rights and only the first is valid (so the neighbor has first rights to buy).

Ran (DH Rishon): If the sale document is seen, this is no less than a gift with Acharayos, to which the law of Bar Metzra applies. Rav Natrunai Ga'on says otherwise, but his words are unclear. The Rashba says that perhaps a gift with Acharayos is different, for it itself is a sale. Here, the gift document is a pure gift. This requires investigation.

Rosh (ibid.): When the law of Bar Metzra does not apply, e.g. the buyer is a neighbor, sometimes a gift is better than a sale. E.g. a seller is more stingy about giving (or retaining a path to a well that was not given with the field) than one who gives a gift. The latter is Mevatel the first only if there is no advantage to the gift.

Rosh (ibid.): If the first was a gift and the second was a sale, the latter is for Acharayos. If a creditor comes to seize the land, Ploni shows both documents. Clearly he acquired from the first date; the latter was written purely to give him more rights.

Rebuttal (Tur, citing the Ramah): In this case there is no problem if he shows both documents! This was a gift, and now he adds Acharayos. This is like adding a tree! (In such a case, all agree that both documents are valid.)

Poskim

Shulchan Aruch (CM 240:1): If Ploni has two documents for one field with different dates, if the first was a gift and the second was a sale, the latter does not Mevatel the former. We can say that it was written to give Acharayos, even if it does not mention Acharayos, because Acharayos Ta'os Sofer. If the first document was a sale and the second is a gift, Ploni acquired from the date of the first. The second was written to evade the law of Bar Metzra.

Rema: Also other advantages of a gift cause that it is not Mevatel the first.

SMA (4) and Gra (5): The Rema did not say that a path is an advantage of a gift because we hold like R. Akiva, that also a seller is generous and includes a path. The Ir Shushan and Be'er ha'Golah say that a path is an advantage. This is incorrect. Bava Basra 71a gives an advantage. (If Reuven said 'give to Ploni a house that can hold 100 barrels', and we found that Reuven owns a house that holds 120, we say that he is happy for Ploni to get a bigger house!)

Rema: He must not show the first document in Beis Din. If it is seen in Beis Din, it is primary.

SMA (6): In this case it is primary regarding the law of Bar Metzra, but in any case Ploni has Acharayos, for the giver accepted this on himself. The Ir Shushan says that even when taking the giver to Beis Din to collect Acharayos he should not show both documents.

Question: Why did Chachamim counsel Ploni how to deprive the neighbor of his right of Bar Metzra?

Answer #1 (Prishah 2): If Ploni would give the price of the field to the seller, and the seller would give it for a gift, the law of Bar Metzra would not apply. Even now that he bought it, in order that he will have Acharayos, it is not improper to evade the law of Bar Metzra. Chachamim counsel him not to show the sale document, lest the neighbor protest "You must do what is straight and good!"

Answer #2 (Nesivos ha'Mishpat, Bi'urim 1): Chachamim allow this only when the gift really was primary. Likewise, Ploni may take from buyers with the sale document only if it truly was a sale.

Rema: Some say that if the second was a sale, even if both were seen in Beis Din they are valid.