The status of a key source of financing for Inmet Mining Corp.'s $4.3-billion Cobre Panama project hinges on efforts to alter Panamanian foreign investment regulations.

Inmet secured Canadian $500 million ($493 million) from a subsidiary of Temasek Holdings Ltd., Singapore's sovereign wealth fund, in March—an investment earmarked largely for the open-pit copper-molybdenum-gold project in Panama.

But whether that investment can actually be put to use will depend on the success of proposed legislation to change Panama's Mineral Resources Code and remove...