If they are acquiring the property upon paying off the contract for deed (with the loan you’re making) then it would be a purchase.
1026.37(a)(9)”(i) Purchase. If the credit is to finance the acquisition of the property identified in paragraph (a)(6) of this section, the creditor shall disclose that the loan is for a “Purchase.””
If they have previously acquired the property (through the contract for deed/land contract) then it would be a refinance.
Let us know if you have other questions.