Tom Sandoval is fighting Ariana Madix as she tries to force a sale of their home ... claiming he loaned her a big chunk of change that he wants back before selling the crib they shared.

According to new legal docs, obtained by TMZ, Tom alleges he loaned Ariana $90,000 and that she hasn't paid him back.

More importantly, Tom wants Ariana to fork over the dough before a judge potentially orders them to sell the house and split the proceeds.

In his response to Ariana's lawsuit, he also says he has a lien on the house that's directly tied to this alleged loan to her -- and he says no sale can go down until this is dealt with.

As we first told you ... Ariana is suing Tom over the Los Angeles house they co-own. They lived there when they were a couple, but Ariana wants to sell it and split the money now that they've broken up as a result of his cheating scandal.

Tom's so far resisted ... and now it looks like he's digging his heels in even further and claiming she needs to settle up with him over this loan before selling the house. As for the other allegations Ariana made in her original suit ... he issues general denials against them.

Remember ... Tom and Ariana bought the place back in 2019 for around $2 million, and like most real estate in Los Angeles it could probably sell for more now.

Tom's also recently said he's offered to buy out Ariana, making a $3.1 million cash offer to keep the place for himself.

Based on their history, though ... this is shaping up to be a big fight.