retainer
noun [ C or U ]
MANAGEMENT, HR uk/rɪˈteɪnər/ usan amount of money that you pay to a lawyer, consultant (= someone paid to give expert advice or training), etc. in order to be sure that they can work for you when you need them to:
The firm retained White as a consultant, paying him a $5,000 retainer.
get/be on/receive a retainer Penn's firm is on a retainer of $15,000 to $20,000 per month.
An elite 15% of all headhunters are on retainer, reaping two-thirds of their pay up front.
an annual/monthly retainer