what does "keep his butler in fucking bath salt"mean?
“Mr. Strike, your attitude astounds me. You undertook to repay Mr. Rokeby monthly, and you are now in arrears to the tune of—”
“I can’t pay you what I haven’t got. If you hold tight, I should be able to give you all of it back. Maybe even in a oner.”
“I’m afraid that simply isn’t good enough. Unless you bring these repayments up to date—”
“Gillespie,” said Strike, his eyes on the bright sky beyond the window, “we both know old Jonny isn’t going to sue his one-legged war-hero son for repayment of a loan that wouldn’t keep his butler in fucking bath salts. I’ll give him back his money, with interest, within the next couple of months, and he can stick it up his arse and set fire to it, if he likes. Tell him that, from me, and now get off my fucking back.”

Question

what does "keep his butler in fucking bath salt"mean?
“Mr. Strike, your attitude astounds me. You undertook to repay Mr. Rokeby monthly, and you are now in arrears to the tune of—”
“I can’t pay you what I haven’t got. If you hold tight, I should be able to give you all of it back. Maybe even in a oner.”
“I’m afraid that simply isn’t good enough. Unless you bring these repayments up to date—”
“Gillespie,” said Strike, his eyes on the bright sky beyond the window, “we both know old Jonny isn’t going to sue his one-legged war-hero son for repayment of a loan that wouldn’t keep his butler in fucking bath salts. I’ll give him back his money, with interest, within the next couple of months, and he can stick it up his arse and set fire to it, if he likes. Tell him that, from me, and now get off my fucking back.”

Answer

It just means that it is a small amount of money. Bath salts (stuff you put in the bath to make the water smell nice) are not expensive. There is also an implication that the butler is homosexual.

"To keep you in" something in this case means to pay for it.

Answer

basically 'not very much'. 'not enough money to pay for his butler's toiletries'.  Bath salts are (I think) something like Alum, that you add to your bath as as relaxant, and are usually scented. They are not very expensive, and the butler reference is reinforcing the fact that he is very rich. The entire phrase means that the loan is trivial to him.