If Soulja Boy doesn't pay what he owes his ex soon, his current $4.2 million dollar debt is going to pop off ... by about $1,000 in daily interest!

Soulja's ex-girlfriend -- who filed a lawsuit against the "Pop Off" rapper as a Jane Doe -- is still looking to collect the huge debt ... and with every passing day, she says the tab is getting bigger and bigger ... with $1,165 in daily interest, according to legal docs obtained by TMZ.

If you don't remember, Soulja's ex was awarded nearly $4 million following a jury trial which found Soulja liable for sexual battery, infliction of emotional distress, and other claims.

This all came as a result of the musician and his assistant's romantic relationship that quickly turned violent.

We broke the story ... his ex is still looking to get her money ... and has been attempting to have Soulja's music residuals sold off to the highest bidder so she can get the multi-million dollar amount she's owed.