Hilchos Maaser Sheini and Neta Revai 11:10

If some produce hasn’t yet reached the time to tithe it when the time for removal arrives, he can still recite the tithe confession. He need not remove such produce.

Hilchos Maaser Sheini and Neta Revai 11:11

If someone has produce far away when the time for removal arrives, he designates the agricultural gifts and transfers them to their recipients by giving them along with real estate. Or, he can give them to a third party to acquire on behalf of their intended recipients. He then recites the tithe confession the next day. The reason underlying this approach is that transferring movable goods along with real estate makes it a gift with weight behind it. One may not, however, transfer tithes via a kinyan chalifin (a manner of acquisition in which some item, such as a pen or a handkerchief, is exchanged) because doing so resembles a sale, and the Torah requires that terumos, maaseros and other agricultural gifts be given, not sold.