I am negotiating to change my job, but the job offer does not include the salary details and I am waiting for that. However, yesterday my new 'employer to be' texted me 'hope to make contact tomorrow.' What should I say? Do I have to reply the text?
If you answer back "Cool. I definitely hope to hear from you", then you are expressing continued explicit interest in the position offered and you are implicitly letting your prospective employer know that the lines of communications between the prospective employer and you are open, at least at your end. In general, respond with an encouraging note to every positive communication. The hiring process is like a machine. The machine can work without lubricant but it works better with lubricant :)
You don't have to say anything. (Although this answer beautifully explains that it's beneficial if you do)
If we look at the message you recieved, albeit somewhat informal, it basically reads "[I] hope to make contact tomorrow", which suggests he is the initiator.
However, you should reply if you are (largely) unavailable that day. Something along the lines of: "Thanks, but please note that I'm in a meeting that will last until 3 o'clock. I'd be happy to talk to you afterwards" - Thanks jan-doggen
He has your contact, and he is your superior (to be), he will make first contact. If she/he doesn't and it gets late in the day, e-mail him/her (I find e-mail to be more formal than SMS) and either ask for a reschedule or a time when he/she is free - this will slyly remind him/her to contact you.
When they do contact you, ensure you ask for salary details and other details which you are missing. If they offer you the job, try and get this as a written confirmation (i.e.: Would you be able to send me this confirmation through e-mail or letter) - this will secure you better than verbal confirmation.