When selecting Credit Matching Gifts to Both in a mailing, thank-you letters are created for the Individual donor and the MG company for the MG company's MG Pay-Cash gift.

Since the MG Pay-Cash gift does not have any information on the Matching Gifts tab (that information is located on the Individual Donor's original gift), then the Matching Gift company will be blank.

Alternative Solution
To add a matching gift company name to a matching gift-pay cash record, create a gift attribute called Matching Gift Company. Make the data type either text (to type in the company name) or constituent (to perform a look-up of the matching gift company constituent record). While this field would need to be completed for each matching gift pledge payment during gift entry, this gift attribute field would be available to merge into letters in Donor Acknowledgement Letters in Mail.