A condition of a payment qualifying for SLfT credit is that a registered landfill site operator must make it. Therefore a payment by a third party to an approved body would not create an entitlement to a credit to SLfT.
There is nothing, however, to prevent a third party from helping to fund or part fund your contribution, so long as that person or organisation is not itself the approved body you propose to donate to.
The approved body must make sure that any third party contributors do not directly benefit from the projects they are helping to fund. You must also record and notify both SEPA and the approved body receiving the contribution about the third party contribution.
As the SLCF is a voluntary fund, other donations are encouraged to provide good causes with access to funding but only contributions from landfill operators qualify for SLfT credit.