Is it permissible to calculate zakat from an amount that my colleague disposed of before it reached me?

Your colleague erred by taking the money without your permission. This money has become a debt owed by him, which he must repay and repent for. It is not permissible for you to count this money as part of your Zakat because the intention for Zakat was not present when it was taken. Waiving a debt owed by a debtor with the intention of it being Zakat is not permissible. One of the conditions for Zakat to be valid is the presence of the intention when paying it to the poor person or their agent, and this condition is not met here.