Is that true ?
As far as I am aware, the money the French Government alone loaned to the US during the revolutionary war (at least two million dollars[0]) far exceeded the value of Washington personal wealth (estimated at $780,000 in 1799 [1], so at the time of his death, not during the war).
And this is not counting all the loans made from other foreign sources (the Spanish Government and private Dutch investors), and the money raised directly by the Continental Congress.
Also, as others have said, it would have been almost impossible to liquidate his assets (his lands and his slaves) during the war - the problem was availability of cash, not wealth.
[0] https://history.state.gov/milestones/1784-1800/loans [1] https://en.wikipedia.org/wiki/Finances_of_George_Washington#...