What's the noun for billable? Here's the word you're looking for.
bill
A written list or inventory. (Now obsolete except in specific senses or set phrases; bill of lading, bill of goods, etc.)
A document, originally sealed; a formal statement or official memorandum. (Now obsolete except with certain qualifying words; bill of health, bill of sale etc.)
“A couple of days later, I received a bill for several thousand dollars, followed soon after by a threatening letter from a debt collector.”
“Every time there was a large bill to be paid, he was short or said he just hadn't made it to the bank in time.”
“Travis smiles and then puts a ten-dollar bill on the table.”
bill
Any of various bladed or pointed hand weapons, originally designating an Anglo-Saxon sword, and later a weapon of infantry, especially in the 14th and 15th centuries, commonly consisting of a broad, heavy, double-edged, hook-shaped blade, with a short pike at the back and another at the top, attached to the end of a long staff.
“I don't think anyone here would advocate an industry that only survives because of a subsidy paid by the billpayer.”
“That's why the next Conservative government will end any additional billpayer subsidy for onshore wind and give local councils the decisive say on any new windfarms.”
“Ask the billpayer before calling premium-rate telephone numbers.”