Is it obligatory to pay the amount agreed upon with the matchmaker (5000) or the amount set by the group manager (3000), especially since the acquaintance was made through the group?

The aforementioned transaction falls under the category of a ju'alah (a contract for a specific service with a stipulated reward). The woman is entitled to the agreed-upon commission (five thousand) for her mediation in the marriage, because the agreement was finalized upon the completion of the work. You are bound by what you initially agreed upon with her, and she is not obliged to accept a smaller commission.