The rules state: Additionally, to be eligible companies must apply to Y Combinator’s core program by completing the application on our website, located at ycombinator.com. Applicants who are accepted to the core program will not be eligible for the $15K grant.
https://docs.google.com/document/d/1u1PhQxw4zNY3Pu0ZViTW-7tdDUOsqQSRDO9pKH6Bq70/edit#bookmark=id.kxlsfxzbs19b
In a sense if you are deemed good enough for the grant and then accepted but then don't accept the YC offer to join, you also seem to lose the $15k?