The answer you need is from the conference, probably from the person that invited you.
But, make sure that this is a reputable conference. I get many "invitations" to less than reputable venues who are trolling for those fees.
For a reputable place, it might depend on why they asked you, specifically. If they want your expertise, then a substitute probably isn't going to satisfy them.
For a reputable place, you can also plead poverty and ask if the fee can be waived. Whether this works or not depends on lots of things, but if they really want you then --- maybe.
But ask them. An affirmative answer from this site does you no good at all.
A request to attend and speak at a conference not in your field is a fairly clear sign that it isn't reputable. The exception would be for superstars who might have something to say about the crossover between fields. For an assistant professor, I'd guess they want your money more than your ideas. Red flag.