The transaction generally has to progress far enough to create real commitment before the application is credible, so the two run together rather than one after the other.
Commitment has to exist before it can be proved
This category creates an uncomfortable ordering problem that applicants often try to avoid. The evidence required is proof of irrevocable commitment, which means the money must already be exposed before the application can succeed, and the application may not succeed. Structuring the transaction so that commitment is real but staged, with documented conditions, is the usual response, and it is a matter for the transaction lawyer as much as anyone else.
In parallel, source-of-funds tracing should begin early because it depends on records held by banks and institutions that respond slowly. Treating the purchase and the application as one project with shared deadlines produces a better result than finishing the deal and then starting the file.