What should I do if the airline provides me a voucher to be used within the next 12 months?  Can the original charge for the airline ticket remain on the grant?

Until we received different guidance from federal sponsors, the general answer is that this should be funded via institutional funds (i.e., should not be placed on a grant) as it is not clear as to whether this credit will be used at some future date to benefit the initial grant in question, or something else.