The definition of market value is "cash" so if there is a "discount" for a cash sale then every comp in your market with financing needs an adj for cash equivalency, not the other way around!
If cash is better, ie: specifically reflecting a change in sale prices, then this is why we all learned how to do the points and interest rate equivalency adj in basic appraisal classes...if your market reflects financing costs more (higher sale price) then you need to be making a market condition adjustment (which would be negative). FNMA does not allow a + financing adj because the measure (equalizer) is already cash and there cannot ever be a + financing adj, it would ALWAYS be negative!
Read the certification...definition of market value items 4 and 5: terms in cash or cash equivalent...