Try to communicate with them in private. You can gently mention this matter and ask them if they forgot the attached money for some reason. When communicating, keep calm and rational, and avoid emotional expression.
If they do forget, they can negotiate with them for an appropriate solution after reminding them. For example, they can choose to make up money on other occasions or express their blessings and apologies in other ways.
If they refuse to pay with this money, you can consider whether it is worthwhile to destroy their relationship for this money. After all, money is just a form, and true friendship and feelings should not be affected by it.
You can also learn from this experience. In the future, if you invite friends to attend the wedding, you can clarify the way of handling money in advance to avoid similar embarrassing situations from happening again.
When dealing with such problems, the most important thing is to remain calm, rational and respectful. Don't ruin the original relationship because of money. After all, friendship and affection are more precious.
Money is essentially a social etiquette and a way to express blessings, and should not be regarded as a debt or transaction. Therefore, when you want to get some money back, try to avoid bringing pressure or discomfort to the other party. If the other party really has difficulties or is unwilling to pay with this money, you should also respect their choice and try to treat this problem with a more tolerant and understanding attitude.
Although in some specific cases, such as the dissolution of marriage through consultation, invalid marriage cases or malicious extortion cases, the possibility of recovering the money can be sought through legal channels, in most cases, the money is regarded as a gift, and once it is given, it cannot be recovered in principle.