We report an in situ scanning tunneling microscopy study on Sn underpotential deposition (UPD) on sulfate-covered Cu(111) electrode surfaces, which represents a system of anion adsorption in an extreme situation in terms of anion-anion and anion-substrate interactions. Owing to the strong sulfate adsorption, the UPD is initiated with "parasitical" adsorption of Sn adatoms almost exclusively along the periphery of the sulfate adlayer, as manifested in the appearance of brightened Moire structure modulation at the terrace edge. A Sn-induced local enhancement of tunneling is offered to explain the brightening of the Moire structure. The initially deposited Sn adatoms displace the sulfate ions from the edge and push themselves toward the interior region of the terrace, maintaining the brightened Moire pattern at the forefront sites. Consequently, a region free of sulfate is provided for further Sn deposition. In addition to terrace edge reshaping, surface alloying is confirmed by the course-dependent anodic stripping, in which the Sn-covered region of the terrace (excluding the forefront sites) becomes fragmented. The surface alloying is favored, in view of strain relief, by the expanded topmost layer of the reconstructed Cu(111). The overall deposition process may be complete within a time window of several minutes. The present work reveals that when anion-substrate and anion-anion interactions are sufficiently strong and comparable to adatom-substrate interaction, the formation of metal adlayer is severely restricted and novel features of UPD are displayed.