不当得利是民法的重要内容之一，而不当得利返还请求权则是不当得利的核心。
Unjustified benefits is one of important civil law content. But, unjustified benefits returns to the core claim of the unjustified benefits then.
债权让与无效时，债务人可以向受让人主张返还已经履行的给付，该权利在性质上是物权返还请求权或者不当得利返还请求权。
When the assignment is invalid, obligor has the right to restitute from assignee those obligor gives to the assignee and in theory the right is the claim of property right or unjust enrichment.
债权让与无效时，债务人可以向受让人主张返还已经履行的给付，该权利在性质上是物权返还请求权或者不当得利返还请求权。
When the assignment is invalid, obligor has the right to restitute from assignee those obligor gives to the assignee and in theory the right is the claim of property right or unjust enrichment.
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