retainer fee; upfront fee
A fee paid in advance to a professional, especially a lawyer, to secure their services. Common in legal contexts.
I paid a retainer fee to the lawyer.
The upfront fee is required at the time of contract.
Compound of 着手 (ちゃくしゅ, 'start, undertake') and 金 (きん, 'money'). The exact historical derivation is uncertain, but the term is conventionally used in legal and professional contexts for an upfront payment.