You sold the call so the premium is your, no matter what happens.
If your short call is assigned, you'll sell the stock for $100 and you'll be done with it. This is highly unlikely since CELG is $98, the strike is $100 and the expiration is 16 months away.
Based on the merger terms you provided, you will receive one share of BMY and $50 for each CELG share that you own. The CELG options will be adjusted to reflect these terms. If assigned, you will be required to deliver 100 shares of BMY and $50 cash. If you are not assigned, you'll own the BMY shares and you'll get to keep the $50.
Here's a coincidence for you. In 2008, Celgene acquired Pharmion. The terms of the deal were:
- Each share of PHRM was converted into the right to receive 0.8367 of a share of CELG common stock, plus $25.00 cash. Cash-in-lieu was provided for fractional CELG shares.
On the date of the merger, all outstanding options were adjusted as follows to requires the receipt or delivery of:
- 83 shares of CELG Common Stock plus $2,536.81 cash ($2,500.00+ $36.81 cash in lieu of 0.67 fractional share of CELG Common Stock (based on a whole-share price of $54.94 for fractional CELG shares)
You can see how the existing PHRM options were adjusted to reflect the acquisition.