Would he be sinful, whoever signed as a witness a waiver without the presence of the second party, and stipulated to the owner of the waiver that if he were summoned to court, he would testify that he signed without the presence of the second party?

The questioner erred in his action, for testimony is not permissible except with personal knowledge and observation, as Allah Almighty says: "And do not pursue that of which you have no knowledge," and His saying: "But only those who testify to the truth knowingly." Therefore, fingerprinting alone is not sufficient without the presence of the other party and personal observation of the event. He must correct his testimony or destroy it.