Who was Flimnap. How did he put a cross to the emperor the enormous expense incurred on Gulliver?

Filmnap is the kingdom's treasurer.

Very often the chief ministers themselves are commanded to show their skill, and to convince the emperor that they have not lost their faculty. Flimnap, the treasurer, is allowed to cut a caper on the straight rope, at least an inch higher than any other lord in the whole empire.

He put across Gulliver's expenses in the following way;

He represented to the emperor “the low condition of his treasury; that he was forced to take up money at a great discount; that exchequer bills would not circulate under nine per cent. below par; that I had cost his majesty above a million and a half of sprugs” (their greatest gold coin, about the bigness of a spangle) “and, upon the whole, that it would be advisable in the emperor to take the first fair occasion of dismissing me.”