Answer: You or your company should pay the $500 and lock the rate.
Step-by-step explanation:
You had already given your customer reasonable assurance that you would lock the rate so this is what they expect. You must therefore do as you have promised to your customer regardless of what the cost would be because the fault here lies with you.
You should therefore pay the $500 unless there is some company policy that allows them to pay it instead. Once done, you will be able to lock in the rate and fulfill your obligation to your customer.