Rapper Future's Baby Mama -- He Lied, He's Ballin

Rapper Future's baby mama #1 is going back to the Future -- for child support, because she believes he swindled her by lying to the judge about his net worth.

TMZ broke the story ... Future and baby mama Jessica Smith struck a child support agreement in which the rapper agreed to pay $1,662 a month for their 10-year-old son ... predicated on his monthly income which he set at $16,516.

But days after they hashed out the agreement, Smith says Future -- real name Nayvadius Wilburn -- went on TV and boasted he carries $25,000 cash on him at all times ... MINIMUM.

Smith immediately went back to court, filing new docs to invalidate their deal and increase his child support obligation. She believes his actual monthly income exceeds $50,000.