Hello Arthur,
You can incinerate the yellow precipitate in an open crucible first, with access to air at about 600-650 deg C, tempering at the same temperature for 1-2 hours, then cover the crucible with a ceramic lid, heat further up to at least 800 deg C, where PdO is decomposed to the metal, holding this temperature for another 1-2 hours and then let cool the covered crucible in the oven, under minimum access of air.
The thus obtained Pd-sponge is readily dissolved with 32% HCl, diluted with half it's volume water, by moderate heating (70-80 deg C) and slowly adding 35% H2O2, diluted with an equal volume of water with constant stirring. Alternatively oxydation can be achieved by the slow addition of an aqueous solution of sodium chlorate (NaClO3)
ClO3(-) + 5 Cl(-) + 6 H(+) => 3 Cl2 + 3 H2O
H2O2 + 2 H(+) + 2 Cl(-) => Cl2 + 2 H2O
The elemental chlorine formed will oxidize the Pd to Pd(II) in PdCl4(2-), and, eventually to some Pd(IV) in PdCl6(2-). It is important, that you always have excessive HCl in the reaction mixture. I am recommending the HCl/chlorine-method for dissolving Pd-sponge or Pd-black, because in my hands dissolution in aqua regia in most cases turned out to be a messy reaction, only difficult to control, which often lead to extensive cleaning/recovering-actions of the work place under the fume hood.
Regards, freechemist
