owning your masters

Hi all. so if were to record your own original songs in someones home studio, and you want to ensure you own your masters so there’s no hassle you could potentially get when you release an album, or get a sync placement. What do you need to have in place to secure your ownership of the master recordings. Is them sending you the project file/wav stems enough even though now you now both have the project file? Is it essential to have a written agreement in place? maybe small purchase of their services (even if it’s just £1) with a receipt needed to prove ownership, even though their intention is to let you record at their studio for free?

Thanks in advance for any info on this matter.