Is what the questioner did, by contacting his former fiancée—who is going through a getting-to-know-you period with another person—considered an injustice to the third party who legitimately proposed to her and whose proposal was accepted by her guardian?

The authentic Sunnah prohibits proposing when a proposal has already been made, if the two suitors have agreed and each is inclined towards the other.

However, if you proposed to her before their agreement, or if they agreed but you were unaware of it, or after the engagement was broken off, then there is no sin upon you. You will not be harmed if she broke off her engagement for your sake, without any instigation from you.

Two points should be noted: 1. What some people do, leaving room for the girl and the suitor to get acquainted, is a bad habit that can lead to corruption and marital failure. The best way to ascertain the character of the suitor or the fiancée is to ask trustworthy individuals. 2. One should be keen on choosing a person of religion and good character when marrying. Religion takes precedence over beauty and lineage, and with it, the family will be stable and the companionship will last.