'The seller wants in return 10 million in a mortgage'
Yes, it needs an article, but it needs to go before the number, and you want to drop 'in'. The seller wants (in return)
a 10 million Forint mortgage.
In this sense 'mortgage' does not mean the loan, it means the right granted by the owner (that's the buyer) to the lender, that allows the lender to repossess the property if the loan is not repaid as agreed.
Typically the buyer gets a loan from a bank, so that the seller can be paid in full. But in this case, no bank is involved, and the seller is simultaneously acting as lender.