Deductions & credits

Thanks, there seems to be confusion around adoption expense.  For example, if my lawyer is content waiting until I can claim the credit when I file in Jan 2018, is it sufficient that I have an "expense/invoice" that is unpaid as documentation for taxes?  I think it is, but can someone help me find this in the tax code/worksheets for the adoption credit so I understand this better?  I'm trying to figure out if I "have" to pay the invoice here in 2016, or can I just wait and pay it in 2018 as I claim the credit?