Within days of Boleslaw's murder, Casimir III of Poland invaded the principality to save Polish merchants and Catholic residents from attacks in Lviv. In June 1340, Casimir returned with a larger army. After four weeks he reached an agreement with local nobles and their leader Dmytro Dedko: in return for their services, local nobles would enjoy protection from the Polish king. However, the agreement was short-lived. The data is sparse, but it seems that Galicia-Volhynia was divided between the Lithuanians  and local nobles . During the winter 1340-1341, the Golden Horde  attacked Poland and reached Lublin as a result of diminished tribute from the principality to the Mongol khan. The raid weakened Polish influence in the principality. Eufemija, Boleslaw's widow, was drowned in the Vistula in winter 1342 to keep her out of the succession disputes. Detko, who managed to play Poles, Lithuanians, and Mongols against each other, disappeared from written sources in 1344. The same year direct conflict between Poland and Lithuania renewed, but soon a peace treaty was signed: Volhynia was assigned to Liubartas and Galicia to Casimir.

What did Casimir do first: returned with a larger army or made an agreement with local nobles?
returned with a larger army