Will capital gains be triggered in an ongoing installment sale if the buyer now wants to transfer that encumbrance to another property he owns?

The buyer has been paying his mortgage on the installment sale for 2+ years but wants to get a construction loan for the property. The lender will not take a subordinate position, so the buyer wants to transfer the current indebtedness to his personal property. Will that trigger capital gains on the gain portion of the installment balance or will gains continue to be reported as principal is paid.