I've never seen a position that the Temple can refuse Hekdesh.
Regarding your scenario, it's an explicit Mishna in Pe'ah 4:7 that - if he consecrated his field before harvesting and the harvest was done while it was Hekdesh - the poor are not entitled to the Pe'ah, even if he then redeems it himself:
הִקְדִּישׁ קָמָה וּפָדָה קָמָה, חַיָּב. עֳמָרִין וּפָדָה עֳמָרִין, חַיָּב. קָמָה וּפָדָה עֳמָרִין, פְּטוּרָה, שֶׁבִּשְׁעַת חוֹבָתָהּ הָיְתָה פְטוּרָה:
One who dedicated standing crop [to the Temple] and redeemed it, he is obligated [to give Peah]. One who dedicated cut sheaves and redeemed them, he is obligated. [One who] dedicated standing crop and redeemed [after it became] cut sheaves, he is exempt, for at the time that it would have become obligated, it was exempt.