What is the ruling regarding someone who doubts that he made a vow to Allah and is unable to fulfill it?

If a person doubts whether a vow was made at all—did he vow or not?—then nothing is incumbent upon him, because the default state is freedom from liability. However, if he is certain of the vow but doubts the specification of what was vowed, then an expiation for an oath (kaffarat yamin) is incumbent upon him.