Does this credit have a $10 value to you?
Yes it does. I can fly to Boston and use it there. I can give it to my fourth cousin's friend's ex-classmate who lives in Boston and would like to use it for me. Etc etc. The fact that I do not find it convenient to do so does not negate its intrinsic value.
Let me ask you a question:
Let's say you are on a flight leaving Israel with 10 Shek in your pocket, and you know that you have no way of using it back home in the USA. You also know that you are not returning to Israel in the foreseeable future [Golus plan]. So, you decide to give the 10 Shek to a Meshulach on the plane who is collecting money. Are you not entitled to deduct that against what you owe Maser?
I imagine you will answer yes.
Well, how is this case different?