Can the IRS ask me to pay tax on profits after cashing out shares BEFORE I have received the funds??

I’m a non US citizen. Through an investment company I bought shares in a US company. The company is in the process of being bought out by a larger company, so my shares have been cashed out for a decent profit. My fund manager had me apply for an ITIN as the payout would mean paying tax to the IRS on my profits. I’ve now been given a notice from the IRS, through my fund manager, invoicing me for the tax on the income, which my fund manager says I have to pay up front before the funds can be released to me. This seems strange to me. I’ve never heard of tax being paid before income is received. My fund manager says it’s something to do with the fact that I don’t have a US bank account and the funds will be immediately transferred overseas to me, beyond the reach of the IRS, which is why they want the tax paid in advance.

I’ve tried to contact the IRS but the notice I have been sent has the last 4 digits of the ITIN missing, appearing as XXXX on the invoice. My fund manager doesn’t have access to the full number either. When I call the IRS I can’t get any answers without the full 9 digit ITIN.

Can anyone tell me if this is a legitimate request by the IRS, or is this some sort of scam? The tax bill is quite substantial and I’m not prepared to pay it until I have proof that everything is legitimate.

Any advice appreciated!