What is the expiation for a vow -if I am unsure about the vow and do not remember whether I made a vow or not, and I cannot fulfill it due to my financial circumstances-?

There is no obligation to fulfill a vow when in doubt about it. However, if the questioner wishes to err on the side of caution, she may do so. If she is unable to fulfill it, then she must offer an expiation for an oath (kaffarat yamin).