It’s a matter of negotiation since they likely have in mind the terms they’re needing.
In the context you’ve described, it sound like we’re essentially talking about the equivalent of illustrations. Both photos and illustrations are typically sold in terms of usage rights rather than outright copyright transfer sales. For example, if I were approached by a magazine publisher for an illustration, they’re unlikely to want to buy the illustration outright. Instead, they’d likely be more interested in a one-time usage right for their publication.
You can likely find some contract examples online, but those usage right might list limited time frames, like between Jan. 1 2020 to Jan. 1 2022. They might list geographical area in which the artwork could be used, like North America. They might list a quantity, like up to 25,000 copies of a magazine. They might specify limited usage in relevant promotions of the magazine or specify whether or not my name appeared as a credit. The contract should also specify whether you’re selling exclusive rights over that time period or whether you’re free to allow others to use those same designs. In your case, though, all of this would be for clothing, not magazines, but it’s still largely the same.
Really, though, the contract can place whatever limitations you feel are needed and that the client agrees upon. The more the client wants, however, the more the price typically goes up. If the client wants to own the artwork, that amounts to transferring the copyright, so that would need to be specified in the contract and priced accordingly.
As for not billing because they’re already drawn, I wish I could get software companies to do business that way. Next time I buy some software from Adobe, I’ll ask them to give me a discount since the software already exists. I doubt they’d agree to that logic, and, perhaps, neither should you. If you drew the designs in the first place, that took time and expertise. You might not want to charge as much as if they were commissioned, but they’re definitely worth something — already drawn or not.
Again, though, it’s about negotiation — what the client needs, what you need and what you’re both willing to compromise and come to an agreement over for a price.