Converting Thick to Thin disk

Hi All,

I have esxi box with 1.8 TB space and it has VM that has HDD that is 1.1 TB, this HDD has only 50 GB used space.
I want to convert this 1.1 TB HDD to thin but max free space on SAN storage I have is 800 GB.
When I try to do storage motion I get error insufficient free space on storage. What are my options?

Thanks...