It's arguably not even extortion. Venezuela secures a loan via promises of discounted iron ore sales to China. They weren't able to produce the promised iron ore so they continue to owe money because they didn't hold up their end of the agreement. This is the normal operation of failing to fulfil your part of an agreement.

If China asked for something outside the deal that they weren't otherwise entitled to, like say "let me build a military base in Venezuela or else I'll recall the debt" then it's extortion.