Rabbi Yehuda says that in the case of one who gives an animal to a heathen to look after, we punish him by compelling him to redeem the animal even up to ten times its value and he must give its whole value to a Kohen.
As a result the offenser has paid out 11 times the value, and has one redeemed donkey - thus the Knas [Net value] is up to 10 times the value of the donkey.
What happens if the heathen refuses to sell? Obviously we do not force him to sell - but what is the Knass? Does he have to give Kohen the value of the donkey? One times the value or up to ten times the value? Why should the offender benefit from the heathe refusing to sell the donkey?
If some months have passed - the top price of 10 times the value is based upon what value: the value at the time he sold it? or at the current value?
Yeshayahu HaKohen Hollander
The Nimukei ha'Griv asks your question, and says that it depends on the possibilities presented in the Gemara (3a) regarding whether ten times is an exact amount or an exageration. If it is an exact amount, than he suggests that one must give the entire amount to the Kohen if the Nochri does not agree. If it merely means that he must ascertain that the Nochri gives him back the animal, he would only have to give its value (see at length).
Regarding redemption based on the value at the time of the sale or current value, the Pri Chadash (in Mayim Chayim on Gitin 44a) states that we go by the animal's current value. However, the Pri Chadash mentions that the Rif seems to conclude otherwise.
All the best,