This is worth being blunt about, because the assumption that a limit field will physically block a sale is where shops get hurt. Fields tell your staff what the agreement was. Whether a person can proceed anyway is a permissions and approval question.
So set both: the limit and terms on the customer record so everyone can see the agreement, and a rule that credit orders are not confirmed without approval. The second one is the part that actually holds.
The report to watch is receivables by customer with ageing. Total receivables tells you almost nothing; the same amount spread over thirty days and concentrated in one customer at a hundred and eighty days are completely different situations, and only the second one is an emergency.