it's an unusual formulation (like a too literal translation from ???), but the only meaning that could make sense would be:

The Agent is obliged to: -participate in vindication of Principal dues from customers.

=

The Agent is obliged to: -participate in recovering from customers whatever they (the customers) owe to the Principal.

Агент је у обавези да учествује у наплати потраживања Налогодавца од муштерија.


You can find "vindication of dues" used to mean "debt recovery", but it's "Polish English" or "Dubai English" ...

http://www.mtmconsulting.com.pl/our-offer/

https://ittihadlegalconsultants.com/about.html



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Note added at 3 days 15 mins (2021-07-12 21:47:17 GMT) Post-grading
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"P.S. The Contract is concluded in Poland."

Mystery solved!