QUESTION: The Gemara explains that although the husband is entitled use the property his wife owns at the time of his marriage (Nichsei Milug), his rights are limited to reaping the Peros (fruits) and using them for personal consumption. He may not sell to another party the rights to reap the fruit.
The Gemara gives two reasons for this limitation. Rava says that the reason the Peros are given to the husband is "Revach Beisa" -- in order to provide for and benefit the entire household, including the wife. If the husband were to sell the Peros, the household would not benefit (only the husband would).
Abaye says that the Rabanan did not permit the husband to sell the Peros because the buyer might not take care of the field, knowing that he has only purchased the rights to reap the fruits of the field and has not purchased the field itself. The buyer will misuse the field and cause damage to the wife's property.
Rava's reason for why the husband may not sell the Peros is unclear. If the husband does not bring in the Peros for the family to eat, but instead he sells the Peros, why will this not be considered "Revach Beisa"? The money he receives from the sale will be used to provide for the needs of the family, and thus the family does benefit from those Peros! The Gemara itself says that if the husband takes the profits of the sale of the Peros and invests them ("Iska"), the income is "Revach Beisa" since the family benefits from his investment. In a similar manner, anytime he sells the Peros the family benefits from the increased income!
Moreover, the Gemara says only that the husband cannot sell "Karka la'Peros" -- he cannot sell the land with regard to reaping the Peros (that is, he cannot give the buyer ownership of the land for the sake of keeping its produce). The Gemara clearly implies that the husband is permitted to sell the Peros after he reaps them as long as he does not sell the land itself. Indeed, there is no requirement for him to feed his family each and every fruit. He is permitted to sell the Peros presumably because the household will benefit from their sale. Why, then, is he not permitted to sell the land as well, if he sells it only with regard to reaping the Peros? The household will benefit from that sale as well!
(a) The RITVA explains that if the husband sells the Peros after he harvests them, certainly some of the fruit will be eaten by members of his household. In contrast, when he sells all of the land with regard to reaping the Peros, and the husband does not even harvest the Peros, it is not possible for them to eat any of the Peros. That is why it is not considered "Revach Beisa."
The Ritva may answer the first question in a similar manner. The Gemara permits investing the profits of the sale (when the land is sold with regard to Peros) only when the profits are invested in land or commodities from which the family will benefit directly. He may not invest the money in an investment which returns money and not a usable commodity.
(b) The ROSH (8:11) writes that if he sells the Peros as he harvests them each year, the profit he earns and invests certainly benefits the entire family and is considered "Revach Beisa." In the case of the Gemara here, however, he sells the land for its Peros for a long period of time (for example, he sells to someone the rights to reap all of the Peros of the land for ten years). Since he receives the value of a few year's Peros at once, he will probably spend in one year all of the profits he has received in return for the produce which the buyer will reap in the coming years. As a result, there will be no "Revach Beisa" from the land during the following years since the money for the sale will already have been spent.
This answers the first question as well. What is the difference between selling "Karka la'Peros" and selling the Peros themselves and investing the profits? If the profits are invested in some type of marketable merchandise, the husband is not going to use up all of the profits at once but will continue to trade with those goods and profit from them each year. If, however, he receives a lump sum of cash (or consumable goods) for the rights to reap the fruits and does not invest it in another commodity, he will probably spend it all on consumable goods (or consume it all, if he received consumable goods) during that year and there will be no further benefit from the Peros of the field in the following years.