It's a fine, so you can't opt to pay it, unfortunately.
It's ill-advised to request that someone who doesn't want to be a witness is called. You don't have to submit any evidence in writing prior to court, so they wouldn't be able to pre-empt what you say, and that usually means it is more risky than it is worth.
Regardless, legally you need to attend the court on that date, or call the court and explain that you cannot attend. Depending on your importance, they may simply allow you not to attend, or they may try to reschedule. If they cannot do either, you'll have to go, or risk the fine.
The guidelines for claimants at employment tribunals reads;
"You may ask a tribunal to issue a witness order, which will summon someone that you want to have at the hearing, even if they do not wish to attend. You must apply in writing well before the hearing, so the court can consider your request and whether the person is required.
You will need to tell the tribunal the name and address of the witness, what the witness will say and why it will help your case and why the witness is not willing to come to the hearing voluntarily.
If the court agrees that the witness is necessary, they will send a witness order, and the witness will be required to attend the court. Non attendance is a criminal offence."