BPHTB only applies to the rights that can be considered 'ownership' (so hak pakai, HGB, HGU, etc.). So while a rental right is in some cases transferrable under the contract, that wouldn't be considered a taxable event for the purposes of BPHTB.

For over contract it should be quite simple in that PPh 4(2) is payable flat at 10% on rental income, so for example if the original contract was 100 million per year then 10 million tax is paid on that. If the over contract then 150 million, then it depends on how that is done - if both the 100 and the 150 change hands, then 10 + 15 million tax is payable, but if the original 100 million is paid and then only the marginal 50 million then the tax is 10 + 5. Since it's a final tax there is no way to reclaim that money, and it's simply straightforward - 10% of the money that changes hands.