1) early scribal error (see W et pauci)...though highly unlikely
2) Mark's Jesus is stressing the expeditious return of the borrowed donkey.
3) Mark's vulgar/vernacular style occasionally produces unexpected verb tenses (as well as asyndeta and anacolutha)
4) Mark is less concerned with "time" in this particular use of the indicative (similar to Mark 1:11 ευδοκησα), and Mark is rather focusing on the aspect of the verb. In this case, he is less concerned with the mere fact that the donkey will "return" but that "Jesus is sending it back". It seems to me to suggest that the donkey will return on its own by Jesus's control. I may be grasping at straws here, but it may parallel with how the Hebrews occasionally let an animal's action determine "what the Lord has said" because they believed that the Lord could control animals (1 Sam. 6:1-12).
When the ark of the Lord had been in Philistine territory seven months, the Philistines called for the priests and the diviners and said, “What shall we do with the ark of the Lord? Tell us how we should send it back to its place.”They answered, “If you return the ark of the god of Israel, do not send it away empty, but by all means send a guilt offering to him. Then you will be healed, and you will know why his hand has not been lifted from you.” The Philistines asked, “What guilt offering should we send to him?” They replied, “Five gold tumors and five gold rats, according to the number of the Philistine rulers, because the same plague has struck both you and your rulers. Make models of the tumors and of the rats that are destroying the country, and pay honor to Israel’s god. Perhaps he will lift his hand from you and your gods and your land. Why do you harden your hearts as the Egyptians and Pharaoh did? When hea treated them harshly, did they not send the Israelites out so they could go on their way? “Now then, get a new cart ready, with two cows that have calved and have never been yoked. Hitch the cows to the cart, but take their calves away and pen them up. Take the ark of the Lord and put it on the cart, and in a chest beside it put the gold objects you are sending back to him as a guilt offering. Send it on its way, but keep watching it. If it goes up to its own territory, toward Beth Shemesh, then the Lord has brought this great disaster on us. But if it does not, then we will know that it was not his hand that struck us and that it happened to us by chance.” So they did this. They took two such cows and hitched them to the cart and penned up their calves. They placed the ark of the Lord on the cart and along with it the chest containing the gold rats and the models of the tumors. Then the cows went straight up toward Beth Shemesh, keeping on the road and lowing all the way; they did not turn to the right or to the left. The rulers of the Philistines followed them as far as the border of Beth Shemesh.